Polyoxymethylene resins are extensively used as a representative engineering plastic in applications including electric and electronic parts, automobile parts and other working parts by virtue of an excellent balance among mechanical properties, chemical resistance, frictional properties, etc., and good moldability. However, in most cases, they have been molded by injection molding.
In recent years, there is an ever-increasing expectation of the application of the polyoxymethylene resins to hollow moldings, such as various containers including fuel tanks of automobiles, by taking advantage of excellent resistance thereof to chemicals, particularly organic solvents. Blow molding is generally used as an efficient method for the production of such hollow moldings. In order to successfully conduct the blow molding, it is necessary to satisfy a requirement that there should occur neither breaking nor uneven section of the molding due to drawdown of a parison, that is, the resin should have a sufficient melt tension. Since, however, polyoxymethylene resin exhibits generally poor melt tension, difficulty has been encountered with its blow-moldability. This is because the limitation of the molecular weight of the polyoxymethylene has made it difficult to produce a high-molecular weight polymer having a satisfactory melt tension. In order to cope with this difficulty, a proposal has been made on a method of improving the blow moldability through an increase in the melt tension by using a branched monomer in the polymerization to introduce a branched or crosslinked structure into the polymer (see, for example, Japanese Patent Publication-B No. 25114/1970). The branched or crosslinked polymer, however, has problems including a high liability to the formation of particulates in gel form in the molding, a deterioration in the appearance of the molding, a high fragility of the molding and a lack of toughness, so that it cannot satisfactorily attain the purpose.
On the other hand, the polyoxymethylene resin has found extensive applications in moldings having shapes with which injection molding cannot cope, such as rods, square timbers, hollow pipes and sheets. In this case as well, the occurrence of drawdown during extrusion molding due to unsatisfactory melt tension of the polyoxymethylene resin made it difficult to successfully conduct the extrusion melding, so that the application was limited to moldings having a relatively small size. Further, since the polyoxymethylene resin has a high crystallinity, voids are liable to occur in extrusion molding products, which brings about such problems that the mechanical strength deteriorates and at least part of the material cannot be used. The use of a polyoxymethylene copolymer resin having a high molecular weight and a suitable crystallinity is considered effective in solving the above-described problems. However, there is a limitation on the increase in the molecular weight of a linear polyoxymethylene resin from the viewpoint of the production technique, so that no linear polyoxymethylene copolymer resin particularly having a sufficiently high molecular weight and a suitable crystallinity is currently available in the art.